This invention, in one of its aspects, pertains to the detection or measurement of hydrogen chloride gas in areas around Space Shuttle launch pads. In another of its aspects the invention relates to a length of stain dosimeter sufficiently sensitive and accurate to measure such gases, even after several hours have elapsed.
Payload customers for the Space Shuttle have recently expressed concerns about the possibility of their payloads at an adjacent pad being contaminated by plume effluents from a shuttle at an active pad as they await launch on an inactive pad. As part of a study to satisfy such concerns a ring of inexpensive dosimeters was deployed around the active pad at the inter-pad distance. Coupled with active measurements of the HCl intrusion through the air handling system on the active pad, it would then be possible to model the expected HCl concentrations at the inactive pad inside the PCR should the plume pass directly over an inactive pad during a future launch.
Initial studies involved the use of paper badge type dosimeters based on modifications of hydrazine-vanillin and bromophenol blue indicators. It was found that rapid fading of the color developed by these systems made their use difficult if not impossible. Following a launch, dosimeters cannot be read for several hours after the exposure. With the date for deployment approaching it was decided to look at the factors affecting fading in such passive dosimeters.
Factors such as different substrates, solvent systems, and possible volatilization of HCl from the badges were studied. Since interaction of the substrate with the dye formulation could be most quickly seen by dropping 100 .mu.l of the dye (indicator reagent) test solution onto a horizontally suspended substrate and watching for color changes at the edge of the spreading drop, this method was employed. Many papers were found to be basic or acidic by using acid or base forms of bromophenol blue (BPB) dye. Even "Chromo 1" (a high purity paper used in paper chromatography) and silica gel thin layer chromatography (TLC) plates exhibited rapid fading of the developed color unless very heavy acid exposures were used. Only when Teflon and glass disks were used could light exposure colors be retained. This observation led to the length of stain (LOS) dosimeters of this invention.
Length of stain dosimeters are known in the art, as exemplified by U.S. Pat. Nos. 4,348,358, 4,489,164, 4,904,449, 4,913,882 and 4,963,324. Of these U.S. Pat. No. 4,348,358 is of interest herein. That patent relates to a length of stain (LOS) dosimeter having a detecting strip disposed in a tube with at least one of its sides spaced from the tube along a substantial portion of the length of the strip. The U.S. Pat. No. 4,348,358 is generally directed to porous detecting strips, such as filter paper, blotting paper, felt, cloth and wicks, although incidentally mentioned are nonporous strips of glass rods, glass and plastics. The nonporous strips are thus equated with the porous strips with no obvious advantages. Tube sizes and strip sizes are also mentioned, but no characteristics or teachings are given leading to their application or implementation.
Commercial passive LOS dosimeters are sensitive only to the extent of being capable of sensing 2 ppm to 20 ppm if the exposure is 8 hours. U.S. Pat. No. 4,348,358, which shows 5 ppm to 25 ppm over an 8 hour period is within this commercial sensitivity range.
To map and quantitate the HCl generated by Shuttle launches, and in the atmosphere within a radius of 1.5 miles from the active pad, a sensitivity of 2 ppm HCl in the atmospheric gases on an exposure of 5 minutes is required. Translating these values into ppm minutes (ppm X minutes), the only dosimeters known are those having sensitivities in the range or 2 ppm to 20 ppm over an eight hour period (960 ppm min to 9600 ppm min), whereas the dosimeter required in launch areas must have a sensitivity of 10 ppm min. Such a dosimeter is provided herein.